A micro-electromechanical element system (MEMS) is a micro-device that incorporates both electrical and mechanical functions. The MEMS is manufactured by a variety of microfabrication techniques. A micro-electromechanical element is first provided on the surface of a substrate, and a protective mask or an encapsulant is then encapsulated to protect the micro-electromechanical element from external damage, thereby obtaining a package structure with a micro-electromechanical element.
Referring to FIG. 1, a cross-sectional diagram of a conventional package structure with a micro-electromechanical element is shown. A micro-electromechanical element 11 that functions as a pressure sensing element is disposed on a substrate 10 of land grid array (LGA) type, and an electrical connecting end 111 of the micro-electromechanical element 11 is connected to an electrical connecting end 101 of the LGA substrate 10 through wire bonding, thereby electrically connecting the micro-electromechanical element 11 and the substrate 10. Finally, a metal cap 12 is formed on the surface of the package substrate 10 to encase the micro-electromechanical element 11 therein. The metal cap 12 protects the micro-electromechanical element 11 from external contamination or damage. One shortcoming of the package structure for the micro-electromechanical element is that the size is too large, which fails to meet the need for smaller, lighter and more compact end products.
Refer to FIG. 2. In order to reduce the overall size of the package structure with the micro-electromechanical element, FINEMEMS Inc. has filed a patent application (US patent application publication number 2006/0185429) relating to a wafer-level pressure sensing package substrate. In the package substrate, a micro-electromechanical element 11 that functions as a pressure sensing element is directly manufactured on a silicon substrate 13, and then a glass cap 14 is bonded on the top of the micro-electromechanical element 11.
However, in the silicon substrate 13 a sensing cavity 131 and vias passing through the entire silicon substrate 13 are formed, so a Through Silicon Via (TSV) technique is required. This technique uses KOH as an etchant to form the vias or grooves.
Compared with the first prior-art structure, the structure disclosed by U.S. patent application publication number 2006/0185429 dramatically reduces the overall volume of the package structure with the micro-electromechanical element, but TSV technique for forming vias and grooves is costly.
As a result, as shown in FIG. 3C, modern MEMS industry has developed a package structure that provides an electrical connection path from a micro-electromechanical sensing element 202 on a silicon substrate 20 to the surface of the package structure by bonding wires 221, such that the size of the package structure with the micro-electromechanical element can be greatly reduced, without the need for TSV technique, and manufacturing cost can also be reduced.
Refer to FIGS. 3A to 3C, which are cross-section diagrams depicting the conventional package structure with the micro-electromechanical element and its manufacturing method. The steps of manufacturing this third conventional structure first requires using a plurality of bonding wires 22 to electrically connect a silicon cap 21 and an electrical connecting pad 201 on the silicon substrate 20; enclosing the silicon cap 21 and the bonding wires 22 with a package layer 23 to protect the bonding wires 22 from external damage; and then removing part of the package layer 23 and the bonding wires 22 by lapping to expose the ends of the bonding wires 22. The locations where the ends of the bonding wires 221 are exposed may not be consistent, because the locations where ends of the bonding wires 221 are exposed depend on the curvature and height of bonding. These two parameters have to be precisely controlled to obtain consistency in the locations where the ends of the bonding wires 221 are exposed.
Therefore, there is a need for a technical solution that makes the locations where the ends of the bonding wires are exposed in the package structure with the micro-electromechanical element more consistent while reducing manufacturing cost.